Three-group zoom lenses are extensively used for various types of cameras due to their enabling the camera to be compact while favorably correcting aberrations. For digital cameras and video cameras, which are rapidly coming into widespread use, particular requirements such as compactness, high quality images, and low distortion are required of a zoom lens due to the use of a solid-state image detection surface (e.g., a CCD array) that is smaller than film sizes used with conventional cameras.
Digital cameras and video cameras primarily use an automatic focusing system, and high-speed automatic focusing is desired. Among zoom lens focusing systems, inner focusing and rear focusing systems are often used because they move lens elements of less mass and are easier to operate due to the lens elements that are moved being nearer the camera body. The present inventor previously disclosed a three-group zoom lens in Japanese Laid Open Patent Application No. H10-293253. The most remarkable feature of the system described in this publication is that the exit pupil is sufficiently distant from the image plane for efficient detection of the light by a CCD detector array. Unlike photographic film, which can detect light that is incident the image plane at nearly any angle, solid-state image detection elements of a conventional CCD array can efficiently detect only the light that is incident the image plane at nearly a right angle. Therefore, one of the optical requirements for imaging an object onto such a CCD array is to ensure that the light is incident at nearly a right angle, regardless of the image height. In other words, one must ensure that the exit pupil is sufficiently distant from the image plane.
Recently, however, CCD arrays have been developed that can receive light most efficiently when the exit pupil is located at a distance, on the object side of the image plane, that is about 5 times the maximum image size (where the maximum image size equals two times the maximum image height). These CCD arrays are actually less efficient in detecting light when the exit pupil is positioned farther away from the image plane. This is disadvantageous for those zoom lenses in which the third lens group is stationary or barely moved during magnification change, as occurs for the zoom lens described in Japanese Laid Open Patent Application No. H10-293253.
On the other hand, a conventional three-group, rear-focus zoom lens is described in Japanese Laid Open Patent Application No. 59-31922. This zoom lens keeps the distance between the second and third lens groups constant during magnification change when the object being focused upon is at a very near distance. However, the exit pupil often comes too close to the, image plane at the wide-angle end for efficient detection of the light. An attempt to keep them apart to a certain degree causes the distance between the second and third lens groups to widen at the wide-angle end when the zoom lens is in focus at infinity, failing to meet a compactness requirement for the zoom lens.
The present inventor""s three-group zoom lens as described in Japanese Laid Open Patent Application No. H11-90762 can change magnification while keeping the distance between the second and third lens groups sufficient. This zoom lens uses only a six lens element configuration by using aspherical surfaces at several predetermined positions, and favorably corrects for aberrations while realizing compactness. In the future CCD arrays to be used with electronic still cameras are expected to have an enormously increased number of pixels. Currently, CCD arrays having more than 300 million pixels have been developed. Accordingly, there is market demand for an increasingly higher resolution of photographic lenses designed for mounting in electronic still cameras. In addition to the requirement for higher resolution, there is also a need for the zoom lens to have a bright image, which imposes a requirement that the F number of the zoom lens be approximately 2.8 or lower. Ensuring sufficient peripheral light to obtain such a bright image generally causes increased chromatic aberration. Since the market now demands high quality, color photographs, it is necessary to provide a bright zoom lens which favorably corrects for chromatic aberration.
The present invention is a three-group zoom lens, especially one for use in digital cameras and video cameras that employ a solid state, image-detecting array. The first object of the invention is to provide a three-group zoom lens having a zoom ratio of 2.5 or more, high-speed focusing, and an exit pupil that is an appropriate distance for newly available CCD detector arrays, namely, with the overall length between the object-side surface of the first lens group to the image plane being less than 6 times the maximum image size (again, where the maximum image size equals two times the maximum image height). A second object of the invention is to provide a zoom lens having an FNO of approximately 2.8 so as to provide a bright image, and which favorably corrects for aberrations, including chromatic aberration, over the entire range of zoom.